Ring laser angular rate sensors, sometimes referred to as ring laser gyros, are well known in the art. A detailed description may be found in the "Background of the Invention" of U.S. Pat. No. 4,597,667, which is incorporated herein by reference.
Briefly, such sensors include a ring laser supported in a block having a plurality of gas containing tunnels. At the intersection of the tunnels are mirrors to define a closed-loop optical path which is traveled by counter-propagating laser beams therein. Practical embodiments of ring laser angular rate sensors usually include a path length controller. The purpose of the path length controller is to maintain a constant path length. Maintaining a constant path length avoids false rotation errors in the usual sensor output. The path length controller function is usually provided by on of the mirrors being attached to a piezoelectric transducer which controls translational movement of the mirror to effect the laser beam path length.
One technique for maintaining a constant path length is detecting the intensity of one or both of the laser beams and controlling the path length of the ring laser such that the intensity of one or both of the beams is at a maximum. U.S. Pat. No. 4,152,071 which issued May 1, 1979 to T. J. Podgorski, and assigned to the assignee of the present invention illustrates a control mechanism and circuitry as just described. Path length transducers for controlling the path length of the ring laser are well known, and particularly described in U.S. Pat. No. 3,581,227, which issued May 25, 1971 to T. J. Podgorski, also assigned to the assignee of the present invention, U.S. Pat. No. 4,383,763, which issued May 17, 1983 to Hutchings et al and U.S. Pat. No. 4,267,478, which issued May 12, 1981 to Bo H. G. Ljung, et al. All these patents are incorporated herein by reference.
In the aforementioned patents, the beam intensity is either detected directly as illustrated in the aforementioned patents, or may be derived from what is referred to as the double beam signal such as that illustrated in U.S. Pat. No. 4,320,974 which issued on March 23, 1982 to Bo H. G. Ljung, and is also incorporated herein by reference.
In path length control systems of the prior art, the path length control finds mirror positioning for which the lasing polygon path length, i.e., the ring laser path length, is an integral number of wavelengths of the desired mode or frequency (spectral line) of the lasing gas. With proper design, the path length control forces the path length traversed by the laser beams to be a value which causes the laser beams to be at maximum power. The properly designed ring laser has a maximum power at transverse modes commonly referred to as an "axial" on "on-axis" modes. There are many longitudinal on-axis modes of operation which satisfy the intended operating condition. Unfortunately, there are other subsidiary parasitic or secondary maximums between different on-axis modes.. These parasitic maximums are sometimes referred to as "off-axis" modes. The corresponding laser power at off-axis modes is less than when the laser is operating at the on-axis modes.
Path length control systems of the prior art are intended to operate at the maximum power or on-axis mode so that laser sensor performance is optimum. Operation of the laser sensor at off-axis modes can lead to an introduction of sensor rotation and performance errors. Unfortunately, prior art path length control systems are unable to distinguish, by themselves, between the absolute maximums and the parasitic maximums corresponding to the on-axis and the off-axis modes, respectively, of laser operation. In order to achieve true maximum power and mode control in prior art systems, additional circuitry must be provided to monitor the beam intensity to force the path length control to operate at the on-axis modes.